In electronic circuits, the data to be processed are generally represented by absolute values of electrical quantities, voltage and currents. Sometimes, however, it is convenient to encode the data by means of a ratio between electrical quantities, rather than by means of the absolute values of these quantities. Advantages are thus achieved in terms of insensitivity to voltage and current reference values. In fact, if, for example, all of the voltages are proportional to a reference value, such as, typically the supply voltage, then the ratio between any two voltages is independent of this reference value. In the case in question, the datum to be processed is a ratio between voltage and current, that is, a resistance. This means that the input datum, or in a broad sense, the input signal, is a resistance and the output datum or signal is also a resistance.
This situation is represented in FIG. 1 which shows a generic electronic circuit EL of which the input includes a resistance Rin and the output includes another resistance Rout. This output resistance Rout can be detected by its connection to a supply voltage Val by means of a load resistance R1. The current flowing through the two resistances R1 and Rout depends upon the value of the output resistance Rout since the resistance R1 is known and constant. A voltage indicative of this current and hence indicative of the output resistance Rout can thus be detected at the terminals of the resistance R1.
The application for which the present invention has been developed is the indication of the fuel level in a motor vehicle. FIG. 2 is a block diagram of a conventional system in which a sensor with a movable float GAL which is disposed in the vehicle's fuel tank causes a resistance Rj to vary in dependence on the fuel level and consequently varies a current Ij flowing through the resistance Rj.
This float sensor is typically connected to an indicator instrument STR with a needle, for example, of the type with crossed coils. The resistance Rj is connected to the common connection point of the two crossed coils with respective resistances R1 and R2 which generate a magnetic field of variable orientation. This orientation corresponds to the orientation adopted by the needle of the instrument STR. Since each of the two spatial components of the magnetic field is proportional to one of the two currents I1 and I2 in the two coils R1 and R2, the angular position of the movable coil and hence of the needle of the instrument STR depends solely upon the ratio between the currents I1 and I2, and hence on the current Ij absorbed by the resistance R.
This ratio remains constant with variations of the reference voltage Val so that the orientation of the magnetic field, and hence of the needle, does not change with the supply voltage Val. The operating principle just explained is generally termed ratiometricity. Any processing of the signal supplied by the float GAL to the indicator instrument STR necessitates consideration of this principle. This processing typically includes filtering which keeps the indication of the instrument STR substantially constant and accurate, in spite of considerable fluctuations about a mean value induced in the float GAL by the movements of the fuel when the vehicle is in motion.
Circuits which process the signal supplied by the float GAL and have the aforementioned objective are known in the art. For example, this objective is typically addressed by the conversion of the resistance Rj of the float GAL into a voltage which may subsequently be converted into digital form. The voltage is then filtered in a conventional analog or possibly digital manner and the resulting signal is converted once more into an output resistance Rout for driving the indicator instrument STR.
FIG. 3 shows a basic diagram of a circuit according to the prior art, from which it can be seen that, in the steady state, the precision of the ratio Rout/Rj, which should be unitary, depends upon a certain number of factors. One of these factors is IR/VR, that is, the ratio between the current IR which flows in the input resistance Rj and the reference voltage VR of an analog/digital converter A/D disposed 30 at the input of a digital filter FIL. At the output of the digital filter FIL there is also a digital/analog converter D/A and a circuit for converting the filtered voltage into an output resistance Rout. This circuit uses an amplifier A to cause an output current IL to flow through a resistance Rs. The precision of the amplifier A and of the resistance Rs also affect the precision of the ratio Rout/Rj as, of course, does the precision of the two converters A/D and D/A.
These factors are not easily controlled, particularly when the circuit shown in FIG. 3 is formed by monolithic integration. The prior art approaches also have the disadvantage of considerable complexity. Moreover, in the approach shown, there are considerable sources of inaccuracy caused by greatly differing resistance ratios.